At present, the inadequacy and the lack of detection techniques of underground mined-out area constrained the development and utilization of underground space. To solve the problem of discontinuity and blind spot detection in underground mined-out area, this paper presents a multi-frequency ultrasonic detecting technology. In this paper, we proposed the multi-frequency detection method to solve the contradiction between the scope of acoustic detection and the intensity of acoustic attenuation, thus ensuring the continuity of measurement scope in the whole detection process. Secondly, we proposed the criterion of judging the strength of the received signal, realizing the automatic switching between frequency transducers. At the same time, the paper presents an omnidirectional detection method with no blind spot. By introducing the ultrasonic reflector, a 360-degree omnidirectional scanning detection for mined-out area is realized. Finally, by superimposing the depth of the scanning position of the probe, the 3D profile of the mined-out area can be created. Compared with other detection techniques, the feasibility and accuracy of the detection technology are verified. The following conclusions can be drawn in this paper:

  1. This technique is feasible and accurate for the detection of underground mined-out area;

  2. The system can automatically switch the probes with various frequencies when detection range obviously changes;

  3. The system is able to form a 3D profile for underground mined-out area.

1. Introduction

With rapid development of the society and economy, the demand for mineral resources has dramatically increased. Underground mining of mineral resources will inevitably lead to the formation of a large number of mined-out areas. The existence of mined-out areas will lead to the deterioration of mining conditions (causing large-scale falling, rock movement and surface subsidence), resulting in serious casualties and equipment damage (Guo, J.et al., 2006). The development of karst brings important impact on enrichment and migration of oil and gas resources, distribution and runoff of groundwater resources, development and distribution of karst-related mineral resources, as well as development of project construction. In addition, karst-related geological disasters are becoming more and more frequent, resulting in immeasurable losses (Jiang, J, 2008; Yuan, X.B., 2007). Underground energy is generally stored in salt rock, non-permeable and porous rock stratum. Salt rock is internationally recognized as the ideal medium for energy (oil, natural gas) storage. 90% of the world's energy (oil, natural gas) repository is built at salt rock medium or abandoned salt mines (Yang, C.H.et al., 2005). Therefore, the detection of underground mined-out areas becomes a particularly important issue.

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